As a way to combat global warming, introduction of new energy such as solar photovoltaic power generation and wind power generation has been promoted in recent years throughout the world. Since outputs of such power generation are affected by the weather, it is predicted that introduction on a large scale will cause problems with operation of power systems such as difficulty in maintaining frequencies and voltages. As a way to solve such problems, installation of large-capacity storage batteries for smoothing output variations, storing surplus power, and load leveling is expected.
A redox flow battery is one of large-capacity storage batteries. In a redox flow battery, a positive electrode electrolyte and a negative electrode electrolyte are supplied to a battery cell having a membrane interposed between a positive electrode and a negative electrode, to charge and discharge the battery. An aqueous solution containing a water-soluble metal ion having a valence which changes by oxidation-reduction is representatively used as the electrolytes, and such a metal ion is used as active material. In recent years, the most widely studied type is a vanadium redox flow battery in which a vanadium (V) ion is used as active material for each of the positive electrode and the negative electrode (for example, Patent Literatures 1 and 2). The vanadium redox flow battery is currently put in practical use and expected to be continuously used in the future.